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Controlling Resistance Digitally - what kinds of components can be used?

Hello All, I've recently posted on another topic, and thanks for the varied replies, which gave me the information I needed.

I've another question regarding digital control of resistance. I have an application where I'd like to add digitally controlled resistance to pre-existing devices, where I can't modify anything except the resistance control itself. I can't alter the circuit in any other way. For an example, think of a "vintage" wah-wah pedal - in this case, pots do wear out so it's acceptable to replace that part - but that part only. One can't redo the circuit to match the pot, as that could alter the tone...

I'm aware that there are digital potentiometers, but I have been finding that in a number of cases these just are not available in the values I might need to match the existing potentiometer, and/or may not be able to handle the current or voltage that might be present (such as in a guitar amplifier).

So my question would be, what other kinds of resistive devices could be used to provide a digitally controllable resistance?

Any hints on this are greatly appreciated.

Thanks! Patrick Keenan

Reply to
Patrick Keenan
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a motorised dual gang pot in a servo circuit ?

an ORP7 (or similar LDR) illuminated by a LED. (pwm at some ultrasonic rate)

an ordinary pot driven by an off-the shelf R.C. servo.

conbine two digital pots to give the range you want.

Bye. Jasen

Reply to
Jasen Betts

--- View in Courier:

A7>-----------+ | IN>------+--[ASW]---------+--->OUT | | | GND>---+-|----+ | | | | A6>----|-+----+ | | | | | | +--[ASW]---[R]---+ | | | | +-|----+ | | | | A5>----|-+----+ | | | | | | +--[ASW]--[2R]---+ | | | | +-|----+ | | | | A4>----|-+----+ | | | | | | +--[ASW]--[4R]---+ | | | | +-|----+ | | | | A3>----|-+----+ | | | | | | +--[ASW]--[8R]---+ | | | | +-|----+ | | | | A2>----|-+----+ | | | | | | +--[ASW]--[16R]--+ | | | | +-|----+ | | | | A1>----|-+----+ | | | | | | +--[ASW]--[32R]--+ | | | | +-|----+ | | | | A0>----|-+----+ | | | | | | +--[ASW]--[64R]--+ | | +------+

Where 'ASW' is an analog switch, or a solid state relay, or even a mechanical relay if you need the low contact resistance.

JF

Reply to
John Fields

All good ideas. One unusual addition is to use a CMOS switch to apply PWM to a fixed resistor, again at some ultrasonic rate. At low duty cycles there is less current through the resistor, so its value appears to be larger. This works especially well for circuits like wah pedals that are essentially low-pass anyway.

For new designs, there is the advantage that you can have as many variable Rs as you want... and _they_all_track_perfectly with a single PWM signal. Works great for phaser/flanger circuits, where each stage adds another sliding notch to the output spectrum.

Credit for this idea goes to Don Lancaster's "CMOS Cookbook"... strongly recommended!

Bob Masta DAQARTA v4.00 Data AcQuisition And Real-Time Analysis

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Reply to
Bob Masta

Thanks!

As it happens, I have an old copy of the CMOS Cookbook downstairs, which some years ago I read until it started to fall apart. Yes, it's an excellent book.

-pk

Reply to
Patrick Keenan

sed

Something that hasn't been mentioned (and may not be practical in your application) is to use a FET in Ohmic mode. If you keep the drain- source voltage low (say, less than the FET threshold voltage), then the analog voltage at the gate changes the channel resistance. You can use a DAC (plus divider, even) to drive the gate voltage.

There is a nice example on page 140 of Horowitz and Hill's _The Art of Electronics_ (2nd edition). They give two variable-gain circuit examples in Figure 3.33 (in the second, they show how to get around the threshold voltage restriction). In their examples, they use a trick to get a LINEAR relationship between gate-voltage and channel resistance. By putting some resistance R between drain and gate and that same resistance between gate and control signal, you get rid of the quadratic term.

Just thought I'd offer that for completeness.

Best! -- Ted

Reply to
Ted Pavlic

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